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Interesting Clouds on Satellite Imagery

Even on a day of high pressure and benign weather across the Ohio Valley, satellite imagery will oftentimes display something of interest. A weakening cold front was draped over the Appalachians during the early morning hours of December 10, 2011, as seen in Figure 1 below.

Figure 1: Satellite imagery valid at 4:15 AM (EST) and surface analysis valid at 4:00 AM (EST) on Dec. 10, 2011. Mean sea level pressure is plotted in yellow contours. Low-level stratocumulus clouds (indicated by dark gray shading) lingered behind a weakening cold front and stretched from northern Alabama/Mississippi into New England. High pressure and drier air was building into the Ohio and Tennessee Valleys.

Behind the front, lingering low-level stratocumulus clouds extended from northern Mississippi and Alabama into New England. This line of clouds slowly progressed to the south and east and quickly began to erode not long after sunrise. However, one area of clouds in particular stubbornly held on and seemed to align itself with the Blue Ridge Mountains between Tennessee and North Carolina, as seen in the satellite animation in Figure 2 below. What was responsible for this curious alignment?

Figure 2: Visible satellite and surface observation loop valid from 9:15 AM to 3:15 PM (EST) on Dec. 10, 2011 and overlaid on a colored topographic map of the Ohio and Tennessee Valleys. For a given observing station, temperature is denoted by the upper-left number, dew point by the lower-left number, and wind speed/direction by the "arrow" (for a more detailed description on decoding a station plot, click here). The band of post-frontal clouds gradually dissipated after sunrise with the exception of near the Blue Ridge Mountains. Snow cover can be seen across northern Illinois, Indiana, and northwestern Ohio.

First we need to take a step back and examine the necessary ingredients for cloud formation. Clouds form when air is cooled to its dew point (the temperature to which air must be cooled for saturation to occur). This most commonly occurs when air is lifted. Air pressure decreases with height, so as a parcel (an imaginary volume) of air rises, as depicted in Figure 3, it continuously encounters lower atmospheric pressure around it and expands. This continuous expansion requires energy, which takes heat away from the parcel. As a result, the parcel of air cools as it rises. Of course, the reverse of this is also true--sinking air warms as it is compressed by the higher atmospheric pressure around it. In thermodynamics, this is referred to as an adiabatic process (a process in which a parcel of air changes temperature due to expansion or compression without exchanging heat with its surroundings). Once a rising parcel of air cools to its dew point, water vapor in the air parcel may condense onto tiny solid or liquid particles called cloud condensation nuclei, resulting in the formation of a cloud. Figure 3: Adiabatic expansion/cooling of a rising parcel of air. Image courtesy of NOAA/NWS JetStream.

There are a number of lifting mechanisms that can result in cloud formation, but there were two that
Figure 4: Cross-sectional depiction of a cold front. The advancing cold air acts like a wedge, forcing the warmer, moister air to rise. This often results in cloud formation. Image courtesy of the COMET Program.
were responsible for the clouds shown in Figure 2: frontal lifting and orographic lifting. Frontal lifting occurs when two air masses with different temperature and moisture characteristics meet. In the situation depicted in Figure 1, a cold front delineated the leading edge of a cold, dry air mass that was displacing a relatively warmer and moister air mass. As shown in the COMET Program animation in Figure 4, cold fronts have a vertical "sloping" structure. The advancing cold air acts like a wedge, forcing the warmer and moister air to rise, which can lead to cloud formation. Atmospheric stability will determine what types of clouds will subsequently form--whether they are convective (the puffy cumulus type) or stratiform (the smooth, layered stratus type). The atmosphere was stable on the morning of December 10th; thus, the lingering post-frontal clouds were more stratiform in nature.

As mentioned earlier, the cold front was weakening as it drifted over the Appalachians, so its ability to lift the air and generate clouds was also diminishing. Consequently, once the sun came up, it did
not take long for the clouds to begin dissipating. However, there was one area of clouds that lingered well after the rest of the band of clouds had disappeared. Was it merely coincidence that this area of clouds practically aligned itself with the Tennessee/North Carolina state line?

The other lifting mechanism at play in this situation was orographic lifting. This type of lift, also known as upslope flow and depicted in Figure 5, occurs when air is forced to rise as it encounters higher terrain. As seen in Figure 6 below, wind barbs in the vicinity of the Appalachians had a northwesterly component. Although these modeled winds were relatively weak (as were the observed winds plotted in Figure 2 above), the flow was from a favorable direction and just strong enough for air to flow up the western side of the mountains. It would appear that the orographic lift in this instance was sufficient to sustain the clouds
Figure 5:Illustration of orographic lifting. Air is forced to rise as it encounters higher terrain, which often results in cloud formation on the windward side of a mountain. Adiabatic compression/warming of descending air generally causes clouds to dissipate on the leeward side of a mountain. Image courtesy of the University of Idaho.
in the Blue Ridge Mountains region, while the rest of the post-frontal clouds gradually dissolved as the cold front weakened and temperatures rose.

Figure 6:Visible satellite valid at 12:31 PM (EST) on Dec. 10, 2011 and overlaid on a colored topographic map of the Ohio and Tennessee Valleys. Also plotted are GFS modeled mean sea level pressure (in cyan contours) and surface wind barbs (in yellow). Lingering clouds over the Blue Ridge Mountains region were sustained by orographic lift, as evidenced by the northwesterly component of the wind barbs near the mountains.

One may wonder why the clouds dissipated further north along the Appalachians (across West Virginia, for instance) when wind direction was also favorable for orographic lifting there. When looking at the surface observations plotted in Figure 2, however, it can be seen that dew points were much lower in West Virginia (in the teens) than in the Blue Ridge Mountains region (in the 20s). Thus, the encroaching dry air over the Ohio Valley helped to scour out the cloud cover over West Virginia.

Michael Kurz, Meteorologist Intern

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Page last modified: December 19, 2011.
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